The Hidden Challenges of Powered Prosthetics

Modern powered prosthetic limbs are transforming the lives of thousands of users, allowing them to walk, climb stairs, or perform tasks with a degree of freedom and autonomy that was previously thought impossible. Yet behind every successful prosthetic device is an intricate engineering challenge: creating powerful systems that match the fluidity of human motion. Power and responsiveness are two important specifications, but prosthetics also must be comfortable and easy to wear. That’s why, for a powered prosthesis, which replaces a limb, selecting a compact, lightweight, silent, and responsive motor is essential for a viable solution.

At Alva Industries, we believe that solving these challenges requires a fresh approach to motor technology. Our slotless SlimTorq™ motors are a great fit for meeting the demanding requirements of an advanced powered prosthesis.

Fitting High Torque into Compact, Lightweight Joints

A powered prosthesis must deliver sufficient torque to move a full leg or arm yet remain light and compact enough to avoid fatigue and restriction.

Typical prosthetic systems, such as above-elbow arms and above-knee legs, must house structural encasements, batteries, motion controllers, motors, and gearing. To achieve performance comparable to peak human capabilities, high requirements are set.

Achieving high torque typically requires large motors and high gear ratios, both of which add bulk and reduce responsiveness.

A key factor in selecting a motor with an optimal power-to-weight ratio is its motor constant density, which describes how effectively a motor converts its physical volume or mass into usable torque per unit of electrical power loss. It combines the motor constant (Km, expressed in Nm/√W) with the motor’s size or weight, typically normalized as Km per cubic centimeter or per kilogram. A high motor constant density means the motor can deliver more torque for the same amount of copper loss and space or weight, indicating both electromagnetic efficiency and compact thermal design. It is also essential for user comfort because it ultimately determines how efficient the motor can be – ensuring optimal battery life and preventing unwanted heat generated by inherent losses.

• SlimTorq™ motors achieve up to 60% copper fill factor, resulting in 33% higher motor constant density than slotted alternatives.
• They provide 139% higher peak torque, enabling lower gear ratios without increasing motor size.
• The result: compact actuators that deliver fast dynamics with minimal weight- an ideal partner with gearing for functional prosthetic elbow, knee, and ankle joints.

Eliminating Cogging, Non-linearity and Vibration

Smooth, silent motion is essential in prosthetics. Any jerkiness or vibration is uncomfortable and serves as a constant reminder of the artificial limb.

During regular operation (low torque), vibrations and pulsations in a motor can be caused by cogging torque from slotted motor armatures, due to their ferromagnetic arrangement. During peak performance (high torque) the extreme loading can result in additional kinematic jerkiness. The necessary high current draw creates extreme magnetic fields in the motor, flooding the motor’s steel laminations with so much magnetic flux that a saturation level is reached at which torque is not produced effectively. Slotless motors eliminate the cogging and saturation phenomena completely.

• SlimTorq™ motors use a slotless FiberPrinted™ stator and a Halbach array rotor to eliminate cogging torque completely.
• The motors exhibit essentially zero saturation and near-perfect sinusoidal back-EMF for quiet, vibration-free performance.
• This ensures prosthetic joints feel more like a natural extension of the body, without distracting hums or jitters.

Enabling Integration of Advanced Actuator Designs

Powered joints often require integrated gearboxes, leadscrews, or sensors. Selecting the optimal combination is fundamental for overcoming most challenges associated with powered prosthetics. The choice of motor will dictate most of the dimensional and performance constraints, which is why it is important to get this choice right first. Frameless motors offer the highest degree of design freedom, yet most frameless motors offer very limited space for such integration due to small rotor inner diameters.

• SlimTorq™ motors offer a 31% larger through-hole than comparable motors.
• This enables innovative actuator topologies, including:
  
  • Screw-based linear actuators with a rotating nut for reduced inertia.
  • Inrunner-based rotary actuators that are 50% shorter and more efficient than conventional outrunners.
• Designers can now place gearboxes, leadscrews, or sensors inside the motor (Quasi Direct Drives), significantly reducing total actuator footprint.

SlimTorq™ Motors Transform What Prosthetics Can Do

With their combination of high torque density, cogging-free operation, and large internal diameter, SlimTorq™ motors enable powered prosthetics to achieve more than just incremental improvements. They offer:

• Higher torque in smaller spaces
• Silent, vibration-free movement
• Compact, integrated actuators
• Reduced energy use and increased battery life
• Greater user comfort and confidence
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The outer diameter and form factor of the SlimTorq™ Motor 51-12-M make it an ideal candidate for leg and arm prosthetic systems. The motor can deliver very high performance in a small package. The 37 mm inner diameter allows for the integration of additional components such as gearing, electronics and other mechatronic elements. Other motors in the current product range extend the possibilities, depending on the need for size and torque.

Beyond catalog motors, Alva’s FiberPrinting™ technology is scalable. Custom diameters and lengths can be calculated quickly using in‑house design tools, enabling prototypes to be simulated and built well beyond the limits of off-the-shelf products.

Current catalog portfolio:

• Diameters: ø25–160 mm
• Lengths: 9–27 mm
• Application-specific winding constants
• Full simulation support and integration guidance
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Whether supporting a child learning to walk again or enabling an athlete to compete independently, Alva’s motors make the difference.

Want to learn more?

Suggested articles:

• Silent Strength: The Superiority of FiberPrinted™ Motors in Exoskeleton Joints
• The Key to Eliminating Cogging Torque with the Right Motor Technology
• Why and Where to Use a Slotless Motor
• The Value of a High Motor Constant
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Download the white paper: SlimTorq™ Motors for Powered Prosthetics to dive deeper into the metrics, performance charts, and actuator topologies behind this new class of slotless motor.